We have developed a sensitive radioimmunossay system for arginine vasopressin (AVP) which we are employing to study AVP metabolism in fetal and newborn sheep. The effects of vasopressinase in maternal and fetal sera are neutralized by a Bentonite serum extraction procedure. We are developing RIA systems for arginine vasotocin (AVT) and oxytocin and a renal plasma membrane AVP receptor assay. With these systems we plan to continue our studies of AVP kinetics and AVP blood production rates in the fetus and newborn and to extend these to AVT. Finally with the receptor system we plan to study the interaction of alpha and beta adrenergic receptors with AVP.